package org.apache.lucene.search;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;
import java.util.PriorityQueue;

import org.apache.lucene.index.IndexReader;
import org.apache.lucene.index.Term;

/**
 * Base rewrite method for collecting only the top terms
 * via a priority queue.
 * @lucene.internal Only public to be accessible by spans package.
 */
public abstract class TopTermsRewrite<Q extends Query> extends TermCollectingRewrite<Q> {

    private final int size;

    /** 
     * Create a TopTermsBooleanQueryRewrite for 
     * at most <code>size</code> terms.
     * <p>
     * NOTE: if {@link BooleanQuery#getMaxClauseCount} is smaller than 
     * <code>size</code>, then it will be used instead. 
     */
    public TopTermsRewrite(int size) {
        this.size = size;
    }

    /** return the maximum priority queue size */
    public int getSize() {
        return size;
    }

    /** return the maximum size of the priority queue (for boolean rewrites this is BooleanQuery#getMaxClauseCount). */
    protected abstract int getMaxSize();

    @Override
    public Q rewrite(final IndexReader reader, final MultiTermQuery query) throws IOException {
        final int maxSize = Math.min(size, getMaxSize());
        final PriorityQueue<ScoreTerm> stQueue = new PriorityQueue<ScoreTerm>();
        collectTerms(reader, query, new TermCollector() {
            public boolean collect(Term t, float boost) {
                // ignore uncompetitive hits
                if (stQueue.size() >= maxSize && boost <= stQueue.peek().boost)
                    return true;
                // add new entry in PQ
                st.term = t;
                st.boost = boost;
                stQueue.offer(st);
                // possibly drop entries from queue
                st = (stQueue.size() > maxSize) ? stQueue.poll() : new ScoreTerm();
                return true;
            }

            // reusable instance
            private ScoreTerm st = new ScoreTerm();
        });

        final Q q = getTopLevelQuery();
        for (final ScoreTerm st : stQueue) {
            addClause(q, st.term, query.getBoost() * st.boost); // add to query
        }
        query.incTotalNumberOfTerms(stQueue.size());

        return q;
    }

    @Override
    public int hashCode() {
        return 31 * size;
    }

    @Override
    public boolean equals(Object obj) {
        if (this == obj)
            return true;
        if (obj == null)
            return false;
        if (getClass() != obj.getClass())
            return false;
        final TopTermsRewrite other = (TopTermsRewrite) obj;
        if (size != other.size)
            return false;
        return true;
    }

    private static class ScoreTerm implements Comparable<ScoreTerm> {
        public Term term;
        public float boost;

        public int compareTo(ScoreTerm other) {
            if (this.boost == other.boost)
                return other.term.compareTo(this.term);
            else
                return Float.compare(this.boost, other.boost);
        }
    }

}
